(eae )) 
received from and through its respective parent, and must 
therefore be of inconceivable complexity.. We can only speak 
in generalities about processes of which so little is known, 
but we cannot be wrong in assuming that sterility is some- 
times due to the fact that the complex architecture of one 
part-nucleus fails in some way to suit the equally complex 
structure of the other. The individuals of an inter-breed- 
ing community form a biological whole, in which selection 
inevitably keeps up a high standard of mutual compati- 
bility between the sexual nuclei. Individuals whose sexual 
nuclei possess a structure which leads to sterile combinations 
with those of other individuals are excluded from contributing 
to the generations of the future. As soon, however, as a 
group of individuals ceases, from any reason, to breed with 
the rest of the species, there is no reason why the compati- 
bility of the sexual nuclei of the two sets should be retained. 
Within each set, selection would work as before and keep 
up a high standard of compatibility ; between the sets, com- 
patibility would only persist as a heritage of past selection, 
gradually diminishing as slight changes of structure in either 
or both of the sets rendered them less and less fitted to 
produce fertile combinations.* 
It is probable that of all the nice adjustments required in 
the living organism, the mutual adjustment of these incon- 
ceivably complex part-nuclei is the most delicate and precise. 
Now, delicately adjusted organs, such as those of sight, rapidly 
become incapable of performing their functions when in any 
species they have been withdrawn from the operation of 
natural selection ; similarly it is suggested, that the adjustment 
of sexual nuclei to each other would sooner or later give way 
* IT must guard against the inference that the only explanation of 
sterility is here set forth. It is indeed maintained that incompatibility 
of the sexual part-nuclei is the inevitable outcome of enduring asyngamy, 
and is the almost certain cause of the sterility of hybrids. And it may 
be suggested that sterility is a result of the combination of two incom- 
patible germ-plasms in the sexual cells of the hybrid. When the 
incompatibility is not strongly marked we can understand how such 
sexual cells may be capable of fertile fusion with the cells of either 
parent, but not with those of another hybrid. 
But short of these ultimate effects it must not be forgotten that there 
are many obscure factors of asyngamy—causes of various kinds which 
interfere with the fusion under normal conditions or entirely prevent the 
meeting of the sexual cells. 
